Nanoindentation Investigation of FIB-Milled Microstructures to Assess Failure Properties of Cement Paste at Microscale

摘要

A mechanical test methodology using Focused Ion Beam (FIB) milled micropillars and nanoindentation is developed to investigate the failure mechanism of cement paste at the level of calcium-silicate-hydrates (C-S-H), the primary binding phase of concrete. Cement paste is a hierarchical material with different levels spanning the range from nanometers to hundreds of micrometers. C-S-H is primarily responsible for strength and other mechanical properties of cement based materials. Therefore, a fundamental understanding and quantification of the failure behavior of C-S-H at microscale is critical for understanding failure at larger scales. Current experimental techniques, however, are unable to reveal how compressive failure is initiated within the cement paste microstructure. Therefore, a need exists for the development of a robust experimental method that can characterize compressive strength and failure modes of C-S-H.;To address this need, a novel methodology - uniaxial compression of cement micropillars is developed in this thesis. Micropillar geometries are fabricated by focused ion beam milling on potential calcium silicate hydrate (C-S-H) locations identified through coupled backscatter electron imaging (BSE) and energy dispersive spectroscopy (EDS) spot analysis. Uniaxial compression testing of these pillars is performed using nanoindentation equipment. The compressive strength of C-S-H (181-715 MPa) measured from micro-compression tests is found to be consistent with values from multiscale damage and molecular dynamic models in literature. Three primary deformation mechanisms at failure were identified; axial splitting, shearing and plastic crushing of the micropillar were mainly observed. Micro-compression experiments on C-S-H micropillars of varying diameters indicated presence of a size effect with strong increase in strength with decreasing diameter. The deformation mode at failure also exhibited size effect: the dominant failure mode changed from axial splitting to crushing as the pillar diameter was decreased. Compressive strength of C-S-H measured from cement pastes with varying w/c ratio, on the other hand, did not show any significant variation, and thus is identified as independent of composition of the cement paste. Overall, the results of this pioneering work provide valuable insight about origin of strength in cementitious materials, and can be incorporated into multiscale strength homogenization and numerical models for better predicting quasi-brittle failure of cement pastes, mortars and concrete.